Field of the Invention
The present invention relates to a surface light source device to be used in lighting equipment, a backlight of a display of an image display device, or the like, and to a technology capable of stably retaining a light distribution control element.
Description of the Background Art
In recent years, the mainstream in a surface light source device to be mainly used in a backlight is to adopt light emitting diodes (LEDs) as light sources. In a direct-type surface light source device where a substrate having a plurality of LEDs mounted thereon is arrayed on a rear surface of the product, a light distribution control element is provided above the LEDs, thereby diffusing a light and irradiating a liquid crystal panel.
Further, the light distribution control element is classified roughly into a circular lens method in which the light distribution control element is arranged so as to cover each of the plurality of LEDs arrayed and arranged linearly, or a cylindrical lens method in which the light distribution control element is arranged so as to cover all of the plurality of LEDs.
In any of the above-mentioned methods, collapse of the positional relationship between the LEDs and the light distribution control element leads to brightness deterioration and color irregularity, and therefore it is important that the surface light source device be provided with structure of retaining the light distribution control element that eliminates the collapse of the positional relationship between the LEDs and the light distribution control element. In order to securely attain joining strength with respect to the substrate without collapsing the positional relationship between the LEDs and the light distribution control element, such a method is generally employed as to bond the light distribution control element to the substrate with use of a dedicated jig.
In Japanese Patent Application Laid-Open No. 2011-44411, for example, a method is disclosed in which the light distribution control element is arranged so as to cover each of the plurality of LEDs. In this method, an example is disclosed in which leg parts of a round stick shape are provided to the light distribution control element, and the leg parts are fixed to the substrate with use of an adhesive.
In Japanese Patent Application Laid-Open No. 2006-286608, for example, a method is disclosed in which the light distribution control element of a cylindrical shape is used, which is arranged so as to cover all of the plurality of LEDs.
In the technology disclosed in Japanese Patent Application Laid-Open No. 2011-44411, the substrate and a light flux controlling member are fixed with an adhesive. However, the substrate produced with aluminum, CEM-3, or the like and the light flux controlling member produced with polymethyl methacrylate (PMMA), polycarbonate (PC), or the like differ in linear expansion coefficient. Therefore, when using the light flux controlling member formed into an elongated shape, that is, the light distribution control element, the adhesive adversely comes off under influence of expansion and contraction due to temperature. Hence, when using the light distribution control element formed into an elongated shape, an adhesive is improper for fixing the substrate and the light distribution control element.
In Japanese Patent Application Laid-Open No. 2006-286608, an optical performance of the light distribution control element formed into an elongated shape is disclosed. However, structure of retaining the substrate having the LEDs mounted thereon and the light distribution control element is not disclosed.